1. Field of the Invention
The present invention relates to wastewater treatment methods and systems, and more particularly, to a method and system for recycling and treating dyeing wastewater efficiently and at low costs, wherein the method and system are effective in solving problems found with effluent chromaticity.
2. Description of the Related Art
As the dyeing industry requires the use of various dyes, surfactants, and chemical agents, dyeing wastewater is characterized by high chromaticity, marked variation in water quality, and low biodegradability. Therefore, dyeing wastewater has to be treated by both biological and chemical processes in order to reduce biochemical oxygen demand (BOD), chemical oxygen demand (COD), suspended solids (SS), and chromaticity of the dyeing wastewater.
A conventional method employed by most dyeing factories in Taiwan to treat dyeing wastewater essentially comprises, as shown in FIG. 1, the steps of: roughly screening the collected dyeing wastewater; delivering the roughly screened dyeing wastewater to a regulating tank for pH adjustment; performing a biological process (activated sludge and biological sedimentation) on the pH-adjusted dyeing wastewater, followed by delivering the processed dyeing wastewater to a sedimentation tank; performing a chemical process in conjunction with chlorination-induced decolorization on the dyeing wastewater to produce an effluent; and discharging the effluent. Hence, the conventional method for treating dyeing wastewater essentially includes activated sludge and chemical coagulation. These two treatment processes, however, end up with a large amount of sludge whose water content is so high that the sludge can only be dehydrated by distillation. As a result, the conventional method for treating dyeing wastewater is energy-intensive and cost-incurring. Besides, the chromaticity of the processed effluent is too high to meet the existing effluent chromaticity standard recommended by the American Dye Manufacturers Institute (ADMI). Therefore, dyeing wastewater decolorization technology has become an environmental protection issue that draws attention from the dyeing industry.
Reverse osmosis (RO), one of the several membrane filtration techniques in use, is regarded by the dyeing industry as one of the best methods for treating recycled water in terms of the resultant water quality. RO uses reverse osmosis membranes (RO membranes) to remove organic matters from water and reduce chromaticity thereof. With RO, the removal rate of COD and chromaticity is at least 90%. However, as dyeing wastewater contains a large amount of suspended substances, if the dyeing wastewater does not undergo a proper pre-treatment process, the RO membranes are subject to concentration polarization and may produce oxidized crystals. Even worse, the efficiency of filtration may dwindle to an incapacitated level because of a cake layer formed on the membrane surface or because of plugged membrane pores. Hence, RO requires expensive apparatuses and incurs high operation costs. More importantly, the concentrated wastewater resulting from RO cannot be treated again by RO membranes for solid-liquid separation; the dyeing industry can only perform multi-stage distillation dehydration on the concentrated wastewater so as for the ions therein to crystallize. The latter process, however, is energy-intensive and time-consuming and thus poses a thorny problem.
Known methods for treating dyeing wastewater also include a Fenton process that employs titanium dioxide (TiO2) and a process that involves using an electric field or a crystallization technique to boost treatment efficacy and reduce the yield of chemical sludge. Nonetheless, the aforesaid processes are focused on the removal of dyeing chromaticity from wastewater but do not offer an efficient and cost-saving solution to treating the resultant concentrated wastewater (or sludge). In consequence, the dyeing industry is confronted with increasingly high costs for environmental protection and improvement. Accordingly, it is imperative for the related industries to employ both clean production technology and contamination control technology, so as to strike a balance between economy and environmental protection, cut production costs, and enhance competitiveness.